1. Field of the Invention
The present invention relates to a weft storing device for looms and, more specifically, to a weft yarn storing device having a control unit for controlling the operation of a motor for driving the mechanical instrumentalities of the weft yarn storing device so as to prevent the free rotation and reverse rotation of the output shaft of the motor in stopping the same.
2. Description of the Prior Art:
The weft yarn storing device has a rotary yarn guide which rotates to pull out a weft yarn from a yarn package by a length necessary for one picking cycle and to wind the weft yarn around a stationary measuring and storing drum in successive loops. The weft yarn thus measured and wound on the measuring and storing drum is held on the measuring and storage drum with a holding pin, which is retracted to release the weft yarn for picking in timed relation with the picking motion of the loom. When the holding pin is retracted to release the weft yarn from the measuring and storing drum, a picking nozzle jets a pressurized picking fluid into a shed of a wrap yarn to pick the measured length of the weft yarn.
The rotary yarn guide is rotated by a motor, which is started and stopped at a high response speed to wind the required length of weft yarn on the measuring and storing drum in a very short time. Accordingly, an induction motor is employed as the motor for driving the rotary yarn guide, and the operating speed of the motor is controlled through voltage-to-frequency (hereinafter abbreviated to "V/F") conversion. The motor is decelerated through a DC dynamic braking system for the following reasons. That is, a DC dynamic braking system, namely, an electrical braking system, entails little mechanical abrasion, is capable of producing a large torque in low rotation speed zone as compared with other electrical braking systems, comprises less components than other electrical braking systems without requiring conductors and capactitors, and DC power is available on the loom because the loom is provided with an inverter for controlling the operating speed of the motor.
However, when DC dynamic brake is applied to stop an induction motor which is controlled through V/F conversion, it is possible, depending on the stopping phase of the rotor of the induction motor, that the rotor is caused to recoil by a reverse torque produced therein by interaction between the braking magnetic field and the permanent magnetic flux of the rotor. If the rotor of the motor for driving the rotary yarn guide is recoiled, a portion of the weft yarn extending from the rotary yarn guide to the circumference of the measuring and storing drum is slackened to reduce the winding tension of the weft yarn in the initial stage of the next weft yarn measuring and storing cycle. Consequently, it is possible that the succeeding loops of the weft yarn are wound over the preceding loops of the weft yarn in the next weft yarn measuring and storing cycle forming overlapping winds, which affects adversely to the unwinding of the weft yarn from the measuring and storing drum and to the picking operation.
Furthermore, in some cases, when the motor for driving the rotary yarn guide is stopped instantaneously during high acceleration, the rotor of the motor runs free by several turns due to step-out to wind the weft yarn on the measuring and storing drum by an excessive length.